The Initial Corrosion Behavior of 20# Steel under the CO2/Aqueous Solution Gas-Liquid Two-Phase Bubble Flow Condition

The initial corrosion behavior of 20# steel under the condition of gas-liquid (CO2/aqueous solution) two-phase bubble flow was studied through weight loss, scanning electron microscopy, energy-dispersive X-ray spectroscopy, and X-ray photoelectron spectroscopy. The results showed that the corrosion rate decreased rapidly when the corrosion time was less than 3 h, increased rapidly, even to 19.4% of the initial corrosion rate, when the corrosion time was from 3 h to 5 h, and then decreased slowly to about 63% of the initial corrosion rate after the corrosion time exceeded 5 h under different CO2 pressure conditions. The corrosion happened first at the defects area with a high activity such as the cross points of scratches, gradually formed corrosion pits, and then extended around until the corrosion products covered the whole pipe wall surface. At the beginning stage of the corrosion process, the corrosion products were composed of acicular corrosion products and a small number of flocculent corrosion products and formed the corrosion product layer with micro-cracks. With the extension of the corrosion time, the spherical corrosion particles started to form on the initial corrosion product layer's surface and gradually covered the initial corrosion product layer completely. The whole corrosion product layer with dual-structure characteristics formed. The inner corrosion product sub-layer was composed of initial corrosion products with columnar characteristics from the cross-section perspective, and the outer corrosion product sub-layer was composed of spherical corrosion products that were relatively dense. There was no obvious interface between the inner columnar sub-layer and the dense outer sub-layer. As time went on, the corrosion product particles with a broccoli shape characteristic formed on the dual-structure corrosion product layer's surface and finally formed the outermost layer of the whole corrosion product layer. In the end, the whole corrosion product layer with three sub-layers formed, namely, the columnar bottom sub-layer, the relatively dense middle sub-layer, and the surface dense sub-layer composed of particles with a broccoli shape. The main components of the corrosion products were Fe, C, and O, and the main phases of the corrosion products were Fe3C, FeCO3, Fe3O4, Fe2O3, and FeOOH.